A significant difficulty in achieving uniform formation of a paper web on a traveling forming surface is the natural tendency of the fibers to flocculate, i.e., to aggregate or coalesce into small fibrous lumps or loose clusters in the slurry. An objective in Fourdrinier machine designs, and particularly the headbox, has been to disperse the fiber networks during the period of flow through the headbox in such a manner that flocculation has the least tendency to occur on the forming wire surface. Prior art solutions have attempted to accomplish this within the headbox of generating turbulence.
A basic limitation in headbox design has been that the means for generating turbulence in fiber suspensions in order to disperse them have been comparatively large scale or macroturbulence generating devices only. With such devices, it is possible to develop small scale or microturbulence only by increasing the intensity of turbulence generated. As will be appreciated by those skilled in the art, the generation of turbulence presents a continuous spectrum with respect to wavelength. However, for purposes of this specification, microturbulence shall generally be considered as that having a wavelength of about 6 millimeters or less, while macroturbulence shall generally be considered as that having a wavelength of about 40 millimeters or greater. Since the turbulence energy is transferred naturally from large to small scales, the higher the intensity the greater will be the rate of energy transfer and hence, the smaller the scales of turbulence sustained. However, a detrimental effect is also produced by an excessive degree of high intensity large scale turbulence, namely, the large waves and free surface disturbances developed in the slurry on the Fourdrinier table. Thus, a general rule of prior art headbox performance has been that the degree of dispersion and level of turbulence in the headbox discharge were closely correlated, i.e., the higher the turbulence level, the better the dispersion.
Accordingly, one could select either a design that produces a highly turbulent, well dispersed discharge, or one that produces a low turbulent, poorly dispersed discharge. Since either a very high level of turbulence or a very low level (and consequent poor dispersion) produce defects in sheet formation on the Fourdrinier machine, the art of headbox design has typically consisted of making a suitable compromise between these extremes. That is, a primary objective of prior art headbox design has been to generate a level of turbulence which was high enough for dispersion, but low enough to avoid free surface defects during the formation period. This compromise is, of course, different for different types of papermaking furnish, fiber consistencies, Fourdrinier table designs, machine speeds, etc. Furthermore, most such prior art compromises sacrifice either the best possible dispersion or the best possible flow pattern on the Fourdrinier wire.
The defects in sheet formation as a result of these extremes in headbox design, i.e., very high or very low turbulence, are even more marked when one employs a Fourdrinier machine wherein all table rolls and foils are replaced by suction boxes. Thus when the turbulence is very low, as for example in the discharge from a conventional rectifier roll type headbox, the formation of the sheet formed by the rapid drainage over suction boxes in the absence of the table roll activity directly reflects the poor dispersion in the discharge jet. On the other hand, when the turbulence is very high, a wave pattern is generated in the free surface of the flow on the wire as a consequence of the turbulence. With rapid drainage of the suspension in this case, the formation of the sheet reflects the mass distribution pattern of these waves. In addition to the free surface wave patterns, excessive turbulence may also entrain air and disrupt the thickened fiber mat which had been deposited earlier, causing formation defects.
Thus, not only are the prior art extremes of headbox characteristics unsuitable, but it is also difficult to find a suitable compromise for a suction box Fourdrinier application.
U.S. Pat. No. 3,939,037 issued to Hill on Feb. 17, 1976 discloses one method of providing a fine scale turbulence without large scale eddies in the discharge jet by passing the fiber suspension through a system of parallel channels of uniform small size, but large in percentage open area. Both of these conditions, uniform small channel size and large exit percentage open area, are critical according to the teachings of Hill. Thus, the largest scales of turbulence developed in the channel flow have the same order of size as the depth of the individual channels. By maintaining the individual channel depth small, the resulting scale of turbulence will be small. It is likewise critical, according to Hill, to have a large exit percentage open area to prevent the development of large scales of turbulence in the zone of discharge. That is, large solid areas between the channels' exits would, according to Hill, result in the generation of large scale turbulence in the wake of those areas. In the Hill concept, the flow channels must change from a large entrance to a small exit size over a substantial distance to allow time for the large scale coarse flow disturbances generated in the wake of the entrance structure to be degraded to the small scale turbulence desired in the discharge jet.
The approach followed by Hill is thus one of attenuating large scale turbulence generated upstream of the headbox throat to sustain the desired level of small scale turbulence at the discharge jet. Because the geometry of the Hill system of parallel channels of uniform small size is fixed, any change in papermachine operating conditions or speed from the original design condition causes the level of small scale turbulence sustained in the discharge jet to move away from the optimum design level. Thus, the solution suggested by Hill offers the papermaker little flexibility in terms of ability to vary either the operating parameters or the speed of the papermachine if he desires to sustain the optimum level of small scale turbulence in the discharge jet.